8035584: ArrayList(c) should avoid inflation if c is empty

Reviewed-by: martin

src/share/classes/java/util/ArrayList.java

@@ -118,11 +118,18 @@ public class ArrayList<E> extends AbstractList<E>
      */
     private static final Object[] EMPTY_ELEMENTDATA = {};
 
+    /**
+     * Shared empty array instance used for default sized empty instances. We
+     * distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when
+     * first element is added.
+     */
+    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
+
     /**
      * The array buffer into which the elements of the ArrayList are stored.
      * The capacity of the ArrayList is the length of this array buffer. Any
-     * empty ArrayList with elementData == EMPTY_ELEMENTDATA will be expanded to
-     * DEFAULT_CAPACITY when the first element is added.
+     * empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
+     * will be expanded to DEFAULT_CAPACITY when the first element is added.
      */
     transient Object[] elementData; // non-private to simplify nested class access
 
@@ -141,19 +148,21 @@ public class ArrayList<E> extends AbstractList<E>
      *         is negative
      */
     public ArrayList(int initialCapacity) {
-        super();
-        if (initialCapacity < 0)
+        if (initialCapacity > 0) {
+            this.elementData = new Object[initialCapacity];
+        } else if (initialCapacity == 0) {
+            this.elementData = EMPTY_ELEMENTDATA;
+        } else {
             throw new IllegalArgumentException("Illegal Capacity: "+
                                                initialCapacity);
-        this.elementData = new Object[initialCapacity];
+        }
     }
 
     /**
      * Constructs an empty list with an initial capacity of ten.
      */
     public ArrayList() {
-        super();
-        this.elementData = EMPTY_ELEMENTDATA;
+        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
     }
 
     /**
@@ -166,10 +175,14 @@ public class ArrayList<E> extends AbstractList<E>
      */
     public ArrayList(Collection<? extends E> c) {
         elementData = c.toArray();
-        size = elementData.length;
-        // c.toArray might (incorrectly) not return Object[] (see 6260652)
-        if (elementData.getClass() != Object[].class)
-            elementData = Arrays.copyOf(elementData, size, Object[].class);
+        if ((size = elementData.length) != 0) {
+            // c.toArray might (incorrectly) not return Object[] (see 6260652)
+            if (elementData.getClass() != Object[].class)
+                elementData = Arrays.copyOf(elementData, size, Object[].class);
+        } else {
+            // replace with empty array.
+            this.elementData = EMPTY_ELEMENTDATA;
+        }
     }
 
     /**
@@ -180,7 +193,9 @@ public class ArrayList<E> extends AbstractList<E>
     public void trimToSize() {
         modCount++;
         if (size < elementData.length) {
-            elementData = Arrays.copyOf(elementData, size);
+            elementData = (size == 0)
+              ? EMPTY_ELEMENTDATA
+              : Arrays.copyOf(elementData, size);
         }
     }
 
@@ -192,11 +207,11 @@ public class ArrayList<E> extends AbstractList<E>
      * @param   minCapacity   the desired minimum capacity
      */
     public void ensureCapacity(int minCapacity) {
-        int minExpand = (elementData != EMPTY_ELEMENTDATA)
-            // any size if real element table
+        int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
+            // any size if not default element table
             ? 0
-            // larger than default for empty table. It's already supposed to be
-            // at default size.
+            // larger than default for default empty table. It's already
+            // supposed to be at default size.
             : DEFAULT_CAPACITY;
 
         if (minCapacity > minExpand) {
@@ -205,7 +220,7 @@ public class ArrayList<E> extends AbstractList<E>
     }
 
     private void ensureCapacityInternal(int minCapacity) {
-        if (elementData == EMPTY_ELEMENTDATA) {
+        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
             minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
         }